Abstract

This study describes clinical and biochemical consequences of the 3243A→G mutation in the tRNA^Leu(UUR) gene of the mitochondrial DNA. Mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS syndrome) is usually caused by this mutation.

Demyelinating polyneuropathy was observed as a novel feature in a patient with the mutation. Based on electrodiagnostic examination the polyneuropathy was defined as being of the demyelinating, mixed (motor more than sensory) type. In a 1 year follow-up an approximately 7% reduction in both the motor and sensory nerve conduction velocities were observed.

The effect and mechanism of action of nicotinamide treatment in a MELAS patient with the 3243A→G was studied. The blood NAD concentration increased linearly, being 24-fold elevated at 6 weeks of treatment. Blood lactate and pyruvate concentration decreased by 50% within three days and 24 h urine lactate content within 2 weeks. A clinical improvement together with a decrease in the lesion volume in magnetic resonance imaging within the first month were observed. Alleviation of the lactate accumulation during the nicotinamide treatment suggested that an increase in the cellular NAD+NADH concentration led to enhancement of the oxidation of reducing equivalents, suggesting that complex I of respiratory chain operates at non-saturating substrate concentration.

Myoblasts cultured from patients carrying the 3243A→G mutation and from controls were used to measure ATP, ADP, catalase and superoxide dismutase activity, population growth, apoptotic cell death and the morphology of cytoskeletal components. ATP and ADP concentrations were decreased, suggesting a decrease in the adenylate pool. The superoxide dismutase and catalase activities were higher than in control cells, suggesting an increased production of reactive oxygen species due to respiratory chain dysfunction. No increase in apoptotic cell death was observed in proliferating myoblasts, but randomization of vimentin filament direction and length was observed and decreased population growth was associated with the mutation.

The results show that the 3243A→G mutation leads to numerous secondary pathophysiological events. Based on the literature and the results of this study, similarities were found between the pathophysiology of 3243A→G mutation and other neurodegenerative diseases and aging.